/Engine/Source/Developer/TextureFormat/Public/Interfaces/ITextureFormat.h
Returns the number of mips that actually carry bulk data for this texture. Nominally the number of total mips, however some platforms have packed mip tails, which means they still have the total number of mips, but the last several are all bundled together for memory savings.
int32 GetNumEncodedMips ( const [FEncodedTextureExtendedData](API\Developer\TextureFormat\Interfaces\FEncodedTextureExtendedData) * InExtendedData ) const